Due to their light weight and high strength, fiber-reinforced plastics (FRPs) in which a reinforcement fiber material is mixed into a resin are used in various industries, e.g., automobile, construction, aviation, etc.
By way of example, with respect to the automobile industry, the above-mentioned fiber-reinforced plastics are employed as structural members of vehicles, e.g., pillars, rockers, underfloor floors, etc., and as non-structural members that require design, e.g., door outer panels, hoods, etc. Attempts have been made to produce fuel-efficient and environment-friendly vehicles by ensuring vehicle strength while reducing its weight.
With respect to the base members that form the above-mentioned structural members and non-structural members, there are those that are planar in shape, those that are three-dimensional with two or more intersecting flat surfaces, those that are three-dimensional with a curved shape or that combine curved surfaces and flat surfaces, and, further, those in which reinforcement ribs are integrally formed with the above, among others.
Such base members are formed from, for example, a material in which a long fiber material that is 50 mm in length or less is unidirectionally oriented in the matrix resin. However, due to their use of long fiber materials, they have a problem in that they have poor in-mold moldability.
This problem is particularly pronounced when the base member integrally comprises a reinforcement member, e.g., the ribs mentioned above, etc. As such, in order to improve moldability, there are methods in which the base member (or at least the ribs thereof) is/are molded from, for example, a resin material in which short fibers that are 1 mm in length or less are randomly oriented within the matrix resin.
In the case of a base member thus molded from short fibers, by reinforcing an appropriate portion of the base member with a continuous fiber-reinforced resin member, a fiber-reinforced plastic partially reinforced with the continuous fiber-reinforced resin member and having favorable strength properties may be produced while also improving the moldability of the base member.
The continuous fiber-reinforced resin member mentioned above comprises a unidirectional member (UD member) in which a fiber material exceeding 50 mm in length is unidirectionally oriented within a matrix resin, or a quasi-isotropic member (a multiaxial laminated member, a fabric comprising warp threads and weft threads, etc.).
In methods for producing such fiber-reinforced plastics comprising a base member, a portion of which is reinforced with a continuous fiber-reinforced resin member, the continuous fiber-reinforced resin member is disposed in the cavity of a mold, and molten resin for the base member is subsequently injected into the cavity, or molten resin is disposed in the cavity and pressed, in order to make the two adhere to each other and integrate them (since both of the above overmold the molten resin for the base member onto the continuous fiber-reinforced resin member, they are referred to as overmolding). It is noted that Patent Literature 1 discloses a method of producing a fiber-reinforced plastic that is partially reinforced with a continuous fiber-reinforced resin member, wherein a preheated continuous fiber-reinforced resin member is disposed in a mold, a molten long fiber thermoplastic material (LFT) is disposed in the mold, and press molding is performed.
However, the present inventors have identified a problem with respect to overmolding, namely that the continuous fiber-reinforced resin member becomes softened by being heated to its melting point or above due to the heat of the base member molten resin, and the continuous fiber forming the continuous fiber-reinforced resin member bends, resulting in a significant drop in its physical properties. Further, in the case of, for example, a continuous fiber-reinforced resin member comprising a UD member, when the continuous fiber bends and its angle is consequently offset by 3 degrees, its physical property (tensile strength) is known to deteriorate by approximately 10%, or by approximately 50% if it is offset by 12 degrees.
Therefore, with regard to fiber-reinforced plastics that comprise a base member that is reinforced, in part or in whole, with a continuous fiber-reinforced resin member, a fiber-reinforced plastic in which a base member and a continuous fiber-reinforced resin member are favorably made to adhere to each other without causing the continuous fibers forming the continuous fiber-reinforced resin member to bend, as well as developing a production method for same are pressing issues in this field.